1. Field of the Invention
The present invention relates to a traction control system for an automotive vehicle and, more particularly, to a traction control system for an automotive vehicle for an automotive vehicle so adapted as to prevent an excessive slip of a driven wheel of the automotive vehicle on a road surface.
2. Description of Related Art
In order to prevent an excessive slip of the driven wheel on a road surface, technology of controlling traction of the automotive vehicle has already been available which is so arranged as to reduce torque to be applied to the driven wheel, for example, during acceleration of the automotive vehicle, when the driven wheel or wheels slip or spin, or spins or spin, to an excessive extent.
The torque to be applied to the driven wheel or wheels is reduced by reducing braking force to be applied to the driven wheel or by reducing torque to be generated by a combustion engine.
When the traction control is performed by applying the braking force to the driven wheel, braking force adjusting means, or the braking force to the driven wheel, is subjected to feedback control so as to allow an actual slip value of the driven wheel to agree with a predetermined target braking value. In particular, recently, it is desired from the viewpoint of responsiveness, the feedback control has been desired to be performed mainly by differentiation control.
It should be noted herein that the target braking value is set on the basis of a certain basic travelling state of the automotive vehicle, e.g. on the basis of a state in which the automotive vehicle is travelling on a flat road, so that a slip value of the driven wheel to be detected when the vehicle slips may be measured as a slip value larger than the actual slip value when the automotive vehicle is travelling on a bad road, particularly on a largely rough road. As a result, the risk may be incurred that the braking force may be applied to the driven wheel to an extent larger than actually necessary, thereby worsening acceleration of the automotive vehicle. It is further noted that, as the target value is so common that it is set on the basis of the state in which the automotive vehicle is travelling straight forwards, it is desired that the slip value of the driven wheel should be maintained at a smaller value when the automotive vehicle is cornering, in order to secure stability of the automotive vehicle. From such a point of view, it is more appropriate to set the braking force to be applied to the driven wheel so as to comply with the particular travelling state of the automotive vehicle, for example, in which the automotive vehicle is travelling on a bad road surface or it is cornering, by changing the target value when the automotive vehicle has started travelling in the particular travelling state.
It is to be noted, however, that when the feedback control is performed mainly on the basis of differentiation control, the differentiation control is not reflected upon a change in the target value, so that it is extremely difficult to provide appropriate braking force so as to comply with the particular travelling state merely by the technique of changing the target value.
Japanese Patent Laid-open Publication (kokai) No. 106,762/1984 proposes that a signal detected by a sensor for sensing an actual slip value of the driven wheel is subjected to filter processing, thereby removing unnecessary noises contaminated by the sensor. In this case, however, a delay in phase may be caused, thereby making precise traction control difficult.